1. Field of the Invention
The present invention relates to impedance conversion ratio setting methods for impedance conversion circuits that are applied to antenna devices or the like, the impedance conversion circuits, and communication terminal apparatuses including the impedance conversion circuits.
2. Description of the Related Art
As wireless communication apparatuses such as cellular phone terminals and the like become compact, antennas are miniaturized and impedance thereof tends to be lower. Accordingly, in the case where matching is attempted, using a reactance element, on a feeder circuit and an antenna having an extremely lower impedance than the feeder circuit, an available band is undesirably narrowed.
Meanwhile, it is frequently carried out to make a single antenna correspond to a plurality of communication systems. In the case where a single antenna corresponds to a communication system of a low band (for example, 800 MHz band) and a high band (for example, 2 GHz band), a fundamental resonant mode and a high-order resonant mode of a single radiation element are used. However, because impedance of a radiation element differs depending on frequencies, there arises a problem that, when a matching circuit with which matching is successfully carried out at one frequency band is provided, matching cannot be successfully carried out at the other frequency.
In order to solve the above problem, an impedance conversion circuit using a transformer circuit in a matching circuit is proposed, as disclosed in Japanese Patent No. 4761009.
Since an antenna and an impedance conversion circuit are required to be compact, inductance of a primary coil and a secondary coil constituting a transformer of the impedance conversion circuit is extremely small so as to be several nH. This causes problems that magnetic flux is not concentrated because a sufficient number of coil turns cannot be obtained and that an effective value of a coupling coefficient is small because a ratio of inductance generated at an input/output portion of the transformer to the total inductance becomes large.
In order for even a small coil to obtain a predetermined (large) coupling coefficient, it is effective to make the shapes of the primary coil and the secondary coil be the same (almost congruent shape) and dispose the primary and secondary coils so that they overlap with each other.
However, in the case where the primary coil and the secondary coils are made to have the same shape, a degree of freedom to define each inductance of the primary coil and the secondary coil is hardly provided. An impedance conversion ratio is defined in accordance with the inductance of the primary coil and the secondary coil of the transformer. Therefore, it is extremely difficult to obtain a predetermined impedance ratio because of the above-mentioned reason.